1. Field of the Invention
The present invention relates to a lens unit, and an image pickup apparatus capable of mounting the lens unit.
2. Description of the Related Art
Recently, in methods of automatic focusing a camera, the most common method is of obtaining a TVAF evaluation value that represents the sharpness of an image from an image signal obtained by a photoelectrical conversion of an object. Focusing is performed by controlling a focus lens position so that the television auto-focus (TVAF) evaluation value peaks (hereinafter, TVAF method).
Generally, the TVAF evaluation value in the TVAF method is generated using a level of a high-frequency content of an image signal extracted by a band-pass filter. This is because, when a normal object is picked-up, the TVAF evaluation value increases as a focus lens comes close to an in-focus position. This is illustrated in FIG. 2, and the point where the level of the TVAF evaluation value peaks is the in-focus position for the object.
As a lens control of the TVAF method, there is an operation of detecting an in-focus direction based on a change of a TVAF evaluation value when a focus lens is minutely moved to back and forth between a close/infinity side as illustrated in FIG. 3 (hereinafter, reciprocating operation). Since the TVAF evaluation value is generated based on the image signal, it is necessary that the reciprocating operation is synchronized with a vertical synchronizing signal of an image pickup element.
Japanese patent Laid-Open No. H11-125860 describes that an interchangeable lens camera system in which a camera unit generates a TVAF evaluation value and transfers the TVAF evaluation value to a lens unit in communication, and in which the lens unit performs a TVAF control.
On the other hand, in case that the TVAF control is performed not in a lens unit as Japanese patent Laid-Open No. H11-125860 but in a camera unit, a driving instruction of the focus lens is generated based on the TVAF evaluation value in the camera unit, and the driving instruction is transmitted to the lens unit. In this case, when a single communication is performed in synchronization with the vertical synchronizing signal as Japanese patent Laid-Open No. H11-125860, it becomes like FIG. 4. In FIG. 4, in a single communication during one vertical synchronizing time period as before, information on a lens position is transmitted from the lens unit to the camera unit, and the driving instruction of the focus lens is transmitted from the camera unit to the lens unit. Accordingly, the camera unit performs an AF control based on focus lens position information obtained from the lens unit within the vertical synchronizing time period, and transmits the focus lens driving instruction in communication within the next vertical synchronizing time period. Therefore, it causes a problem because the focus lens driving instruction cannot be immediately transmitted based on the lens position information obtained from the lens unit thereby introducing a control cycle delay (responsiveness delay of AF). Further, in case of performing a control of a stop included in the lens unit on the basis of an instruction from the camera unit, the responsiveness of the auto-focus (AF) may deteriorate due to a load of an auto-exposure (AE) control for generating a stop control instruction.